The present disclosure relates generally to energy absorbent occupant protection systems in vehicles and, more particularly, to a method and system for achieving a constant average impact deceleration independent of vehicle load.
Existing motor vehicle structures deliver a specific crush force/deflection trace for an impact event. With such “fixed” structures, changes in the mass of the cargo carried by the vehicle affect the amount of crush, but do not have any significant effect in the crush force that the structure generates at a particular point during the crush. Since the trace of crush force versus crush distance does not vary, while the total loaded vehicle mass can vary for a given vehicle, the vehicle deceleration during an impact can also vary.
However, present occupant restraint systems are designed to provide maximum benefit for a particular vehicle impact pulse corresponding to a specific loaded vehicle mass. As a result, if the loaded vehicle mass is significantly changed, thus resulting in a corresponding significant change in average vehicle deceleration (inversely proportional to the vehicle mass), then the overall effectiveness of the occupant restraint system will be altered. If a vehicle were carrying a “lighter than normal” load (e.g., such as a nearly empty fuel tank, no vehicle cargo, and a single, light driver), then given a constant crush force, the magnitude of the average vehicle deceleration during an impact will be increased with respect to the value at the designed mass, due to the relative decrease in loaded vehicle mass.
On the other hand, if a vehicle were carrying a “heavier than normal load (e.g., a full fuel tank, a heavy cargo load, and five adult passengers), then the relative increase in mass results in a relative decrease in vehicle (and thus belted occupant) deceleration. Although a decrease in average vehicle deceleration is not, by itself, an undesirable condition, it is equally true that a vehicle with increased mass has greater kinetic energy. Therefore, with greater kinetic energy and constant crush force of the vehicle structures, there is also an increase in crush distance.